The doctest module searches a module's docstrings for text that looks
like an interactive Python session, then executes all such sessions to verify
they still work exactly as shown. Here's a complete but small example:

"""
This is module example.
Example supplies one function, factorial. For example,
>>> factorial(5)
120
"""
def factorial(n):
"""Return the factorial of n, an exact integer >= 0.
If the result is small enough to fit in an int, return an int.
Else return a long.
>>> [factorial(n) for n in range(6)]
[1, 1, 2, 6, 24, 120]
>>> [factorial(long(n)) for n in range(6)]
[1, 1, 2, 6, 24, 120]
>>> factorial(30)
265252859812191058636308480000000L
>>> factorial(30L)
265252859812191058636308480000000L
>>> factorial(-1)
Traceback (most recent call last):
...
ValueError: n must be >= 0
Factorials of floats are OK, but the float must be an exact integer:
>>> factorial(30.1)
Traceback (most recent call last):
...
ValueError: n must be exact integer
>>> factorial(30.0)
265252859812191058636308480000000L
It must also not be ridiculously large:
>>> factorial(1e100)
Traceback (most recent call last):
...
OverflowError: n too large
"""

import math
if not n >= 0:
raise ValueError("n must be >= 0")
if math.floor(n) != n:
raise ValueError("n must be exact integer")
if n+1 == n: # e.g., 1e300
raise OverflowError("n too large")
result = 1
factor = 2
while factor <= n:
try:
result *= factor
except OverflowError:
result *= long(factor)
factor += 1
return result
def _test():
import doctest, example
return doctest.testmod(example)
if __name__ == "__main__":
_test()

If you run example.py directly from the command line, doctest works
its magic:

$ python example.py
$

There's no output! That's normal, and it means all the examples worked.
Pass -v to the script, and doctest prints a detailed log
of what it's trying, and prints a summary at the end:

$ python example.py -v
Running example.__doc__
Trying: factorial(5)
Expecting: 120
ok
0 of 1 examples failed in example.__doc__
Running example.factorial.__doc__
Trying: [factorial(n) for n in range(6)]
Expecting: [1, 1, 2, 6, 24, 120]
ok
Trying: [factorial(long(n)) for n in range(6)]
Expecting: [1, 1, 2, 6, 24, 120]
ok
Trying: factorial(30)
Expecting: 265252859812191058636308480000000L
ok

And so on, eventually ending with:

Trying: factorial(1e100)
Expecting:
Traceback (most recent call last):
...
OverflowError: n too large
ok
0 of 8 examples failed in example.factorial.__doc__
2 items passed all tests:
1 tests in example
8 tests in example.factorial
9 tests in 2 items.
9 passed and 0 failed.
Test passed.
$

That's all you need to know to start making productive use of doctest! Jump
in. The docstrings in doctest.py contain detailed information about all
aspects of doctest, and we'll just cover the more important points here.